The invention relates generally to chemical sensors and more particularly to carbon monoxide sensors.
The reaction of Carbon Monoxide (CO) with a variety of molybdenum compounds was reported as early as 1910 [C. Zenghelis, Z. Anal. Chem, 40, 429, (1910)] and the literature was reviewed in 1935 by J. Schmidt, "Das Kohlenoxyd" Akad Verlag, Leipzig, p. 186, (1935). It is further presented in "Spot Tests in Inorganic Analysis" by F. Feigel, V. Anger, R. Oesper, Elsevier Publishing Company, New York, p. 168 (1972).
The three (3) basic equations for the palladium catalyzed reduction of molybdenum by carbon monoxide are: EQU Mo.sup.+6 +CO.fwdarw.Mo.sup.+3 +CO.sub.2 ( 1) EQU Pd.sup.+2 +CO.fwdarw.Pd.sup.0 +CO.sub.2 ( 2) EQU Pd.sup.0 +Mo.sup.+6 .fwdarw.Pd.sup.+2 +Mo.sup.+3 ( 3)
The reaction in Equation (1) is very slow and, therefore, a catalyst (Pd.sup.+2) is used. The Pd.sup.+2, even in very small quantities, adsorbs and simultaneously gets reduced by CO as shown in Equation (2). Pd.sup.0, in turn, enters into the reaction, Equation (3), producing molybdenum blue. Thus, a slightly yellow solution is changed to a blue color with the intensity of the blue color being directly related to CO exposure. As presented in Equations (1), (2) and (3), the reaction is not reversible and, therefore, has limited application in sensor technology.
To make the system reversible, therefore, there must be a secondary reaction which converts Mo.sup.+3 back to Mo.sup.+6, i.e., an oxidizer must be present. A successful reversing agent must meet the following two important criteria: (a) the regeneration reaction should be fast, and (b) the regenerating agent itself should quickly revert back to the starting state for the next cycle.
A reversible CO sensor is shown by Shuler et al., U.S. Pat. No. 4,043,934 which has a Mo, W or V color forming agent, Pd catalyst and Cu, Ni or Fe reversing agent. The sensing reagent is deposited on an inert carrier which is hydrophilic or contains water or OH.sup.- groups, e.g. silica gel, alumina, polymeric alcohol, polyglycol, cellulose, glass wool and sponges.
M. K. Goldstein in U.S. Pat. No. 5,063,164 describes a biomimetic sensor for detecting the presence of airborne toxins, including CO. That patent suggests several possible chemical recipes for this type of sensor, but does not address the criteria or requirements for a successful reversible sensor; nor does it address the chemistry or mechanisms to make the CO sensor completely specific.
Goldstein shows a solid state CO sensor having five components: (1) palladium salt, (2) molybdenum and/or tungsten salt or acid salt, (3) copper salt, (4) cyclodextrin molecular encapsulant which encapsulates at least one but not all of the other components, and (5) chloride salt, all impregnated into a substrate. The Mo,W/Pd/Cu system is as in Shuler. The improvement is the encapsulant which extends sensor lifetime. An excess of chloride ions are also provided to extend lifetime.
Goldstein does not reveal (a) how fast the reverse reaction occurs, or (b) whether it can stand a drastic environment like 100% CO. Goldstein uses Cu.sup.+2 salts as the reversing agent, Cu.sup.+2 and Cu.sup.+ ions are very stable at ambient atmospheric conditions. Therefore, the Cu.sup.+2 /Cu.sup.+ pair does not fully meet the criteria of a successful reversing agent for a deadly toxic gas like CO.